This invention generally relates to an improved electromagnetic interference (EMI) shield. More particularly, the present invention relates to a shield that is easily removable, compatible with single or multi-compartment shield designs, thin in profile, lightweight, and low in cost. This solution is particularly advantageous for use in small electronic devices, such as cellular phones and laptop computers, among others.
EMI shields limit electromagnetic radiation from entering or exiting sections of a printed circuit board (PCB) containing electronic components. A common type of EMI shield is known as a xe2x80x9ccanxe2x80x9d. A can is soldered to the ground trace on a PCB, directly over the electrical components that need to be shielded. Such cans offer extremely high levels of shielding effectiveness, and are typically very reliable. They are often installed in a fully automated fashion via a surface mount technology (SMT) process at the same time the components themselves are installed onto the PCB, using solder paste and a reflow process. The cans themselves are produced by stamping, drawing, folding, or other forming process, and are typically made from metal. This metal is often plated to enhance solderability and protect against oxidation or corrosion. A soldered can may be a very cost-effective solution for providing EMI shielding on a PCB, and is often the shielding method of choice for use in small portable devices such as cellular phones.
There are several drawbacks to using soldered cans, however. One such drawback is that cans are very difficult to remove, once soldered down to a PCB. This fact can prevent easy repairability or inspection of components underneath the cans, which can significantly add to costs in the manufacturing process or during repair. In addition, the cans may impede proper heat flow to these components during the reflow process, sometimes leading to improperly soldered joints. In addition to these issues, other problems with cans exist: they are difficult to produce in multi-compartment designs, where the compartments serve to isolate components or groups of components from each other on a PCB; they are typically relatively heavy since they are made of dense metals such as steel; they typically require clearance over the components they are covering, so they can add significantly to the overall thickness of the electronic package; they are difficult to make with tight flatness tolerances, especially as the cans become large, which can lead to soldering difficulties; they can be difficult to make with tight X-Y dimensional tolerances, which often forces the need for large, mating ground traces to solder to the cans. Cans thus can often consume large amounts of space on a PCB design, which is undesirable as cellular phones continue to get smaller and smaller.
To address problems with repairability of components, open-topped cans have been developed with snap-on or adhesive-backed lids. Such cans are often referred to as xe2x80x98fencesxe2x80x99 or xe2x80x98wallsxe2x80x99 and are affixed to the PCB as a single part in a similar fashion to standard cans, with the lids attached to the fences either before or after the SMT process. An example is found in U.S. Pat. No. 6,169,665. Such removable lids are not always mechanically and electrically reliable, however, especially when made in multi-compartment designs. The multi-compartment, single-part fences are also costly and suffer from some of the same ground trace width and weight problems as cans without removable lids. In addition, flatness issues are of major concern to properly soldering these fences to the PCB, particularly as the compartments get large in size and number.
Other removable shields are used in the industry to resolve some of the issues associated with cans, particularly in multi-compartment designs. These solutions can take several different forms, but a commonly used multi-compartment shield is a metal or metallized plastic cover, fastened to the PCB with screws. Often, a conformable EMI gasket is required at the interface between the shield and the PCB (since solder is not used), to promote more uniform electrical contact across the joint. This type of shielding solution affords the designer much more flexibility in accommodating multi-compartment designs, and gives the benefit of easy accessibility to the PCB during the SMT and manufacturing process.
On the downside, such gasketed shielding solutions have problems of their own. Many screws or other removable fasteners are typically required to adequately compress the EMI gaskets. Thus, the shields must be made from stiff metal or plastic. These requirements add weight, thickness, extra loose parts, manufacturing time, and, ultimately, cost to such designs. The gaskets used with such shields are also subject to damage, which may often lead to short-circuits of nearby electrical components. It is also time-consuming, difficult, and expensive to install or remove all of the fasteners holding the shield in place.
U.S. Pat. No. 6,051,781 describes another type of shield that removably snaps into clips placed around the electrical components on a PCB. Because the clips snap around the edges of the shield, they can only accommodate a single compartment shield design, since internal compartment walls have no edge. These clips, since they are intricate in shape, can be extremely costly and are required in large numbers to provide proper shielding effectiveness in large, multi-compartment designs. In addition to cost, however, this device suffers from many of the same disadvantages mentioned above, such as ground trace width, thickness, and weight, among others.
What has not heretofore been provided, and what is needed, is a shield that is removable and yet compatible with single or multi-compartment designs, thin in profile, lightweight, and low in cost.
The present invention provides an apparatus having a substrate with at least one electrical component disposed on it; a plurality of discrete electrically conductive fastening units disposed in a pattern on the substrate surrounding the at least one electrical component; a shield comprising a dielectric material layer having an inner surface and an outer surface and an electrically conductive layer over at least one of the inner and outer surface; a plurality of apertures formed in the shield such that the apertures correspond to the pattern of the electrically conductive fastening units; wherein at least one of the apertures has a contact region and wherein both the dielectric material layer and the electrically conductive layer of the shield at the contact region of the aperture are deflectable to the extent necessary to allow the contact region to engage and retain the electrically conductive fastening unit; and wherein the electrically conductive layer of the shield at the contact region is in electrical contact with the electrically conductive fastening unit.
In another aspect, the present invention provides an electromagnetic interference (EMI) shield for a substrate having at least one electrical component disposed thereon and a plurality of discrete electrically conductive fastening units disposed in a pattern on the substrate surrounding the at least one electronic component, the EMI shield comprising: a dielectric material layer having an inner surface and an outer surface; an electrically conductive layer over at least one of the inner and outer surface; a plurality of apertures formed in the shield such that the apertures correspond to the pattern of the electrically conductive fastening units; wherein at least one of the apertures has a contact region and wherein both the dielectric material layer and the electrically conductive layer of the shield at the contact region of the aperture are deflectable to the extent necessary to allow the contact region to engage and retain the electrically conductive fastening unit; and wherein the electrically conductive layer of the EMI shield at the contact region is in electrical contact with the electrically conductive fastening unit.
In still another aspect, the invention provides an apparatus having a substrate with at least one electrical component disposed thereon; a plurality of discrete electrically conductive fastening units disposed in a pattern on the substrate surrounding the at least one electrical component; a shield consisting essentially of an electrically conductive material; a plurality of apertures formed in the shield such that the apertures correspond to the pattern of the electrically conductive fastening units; wherein at least one of the apertures has a contact region and wherein said electrically conductive material of said shield at the contact region is deflectable to the extent necessary to allow the contact region to engage and retain the electrically conductive fastening unit; and wherein the electrically conductive material of the shield at the contact region is in electrical contact with the electrically conductive fastening unit.
In yet another aspect, the invention provides an apparatus having a substrate with at least one electrical component disposed on it; a plurality of solder spheres disposed on the substrate surrounding the at least one electrical component; an EMI shield comprising at least one compartment adapted to cover the at least one electrical component, the EMI shield further comprising a dielectric material layer having an inner surface and an outer surface and an electrically conductive layer over at least one of the inner and outer surface; and wherein the electrically conductive layer of the EMI shield is in electrical contact with at least one of the solder spheres, and wherein the EMI shield and the solder spheres combine to limit electromagnetic radiation from entering or exiting the at least one compartment.
The invention also includes other aspects in which the electrically conductive fastening unit is a solder sphere, the substrate has a ground trace disposed on it and the solder sphere is soldered to the ground trace on the substrate, the solder sphere is in interference contact with the contact region of the shield, the electrically conductive layer is disposed on the outer surface of the dielectric material layer, the electrically conductive layer is selected from the group consisting of aluminum, tin, gold, nickel, silver, copper and combinations and alloys thereof, the electrically conductive layer is foil, the electrically conductive layer is formed by a process selected from the group consisting of sputtering, vacuum or vapor deposition, electroless plating, and electrolytic plating, the electrically conductive layer is a dielectric material containing conductive particles, the substrate has a plurality of electrical components and the shield comprises a plurality of compartments adapted to cover the plurality of electrical components, the shield has a plurality of apertures formed in it, where the apertures are formed as the electrically conductive fastening unit penetrates the shield. These aspects of the invention may be used in combination with one another or separately in the inventive devices mentioned above.